In order to obtain a large screen image, there has been used a projection display that allows light from a light source to illuminate a small light valve forming an image in accordance with a picture signal and this optical image to be projected on a screen in a magnified state through a projection lens. In such a projection display, an active-matrix, transmission liquid crystal light valve that modulates light by utilizing polarization has been employed as the light valve in wide-spread practical use. The transmission liquid crystal light valve includes a liquid crystal cell formed by sealing 90-degree twisted nematic liquid crystal between two glass substrates opposing each other and two polarizing films disposed on both sides of the liquid crystal cell so that their transmission axes are orthogonal to each other. By the use of such a transmission liquid crystal light valve, it becomes possible to miniaturize projection displays and also to obtain high-brightness, high-quality large screen images. However, the problem has been noted that, when a light valve having a fixed-pixel structure is used, a pixel grid on a projected image is conspicuous, resulting in the deteriorated image quality. In particular, the numerical aperture, i.e., a ratio of an effective pixel area to a whole pixel area, of transmission light valves is in the range of 40% to 70%, which is lower than that of reflection light valves. Therefore, when a transmission light valve is used, a pixel grid as ineffective portions (such as wirings and TFT (Thin Film Transistor) parts) of respective pixels is conspicuous. In this case, the pixel grid becomes more conspicuous with the increase in the angle of view (due to the increase in the screen on which the image is projected or the decrease in the distance from a viewer to the screen).
In order to make the pixel grid inconspicuous, there has been proposed disposing a pixel separation optical element including a birefringent element for separating incident light into ordinary rays and extraordinary rays and a quarter-wave plate for recovering the polarization between a light valve and a projection lens, so that respective pixels on a projected image are separated at least in two directions (see JP 64(1989)-3834 U and JP 11(1999)-167105 A, for example).
FIG. 9 shows a schematic configuration of a conventional projection display. As shown in FIG. 9, the conventional projection display includes: a light source 1; a condenser lens 2 for gathering light from the light source 1; a liquid crystal light valve 3 that is illuminated by the gathered light from the condenser lens 2 and forms an image in accordance with a picture signal; a first quarter-wave plate 4 for converting linearly polarized light emitted from the liquid crystal light valve 3 into circularly polarized light; a first birefringent plate 5 for separating the light that has been converted into circularly polarized light by the first quarter-wave plate 4 into two circularly polarized light beams that are spatially separated and then converting them into linearly polarized light beams orthogonal to each other; a second quarter-wave plate 6 for converting the two linearly polarized light beams that are spatially separated into circularly polarized light beams; a second birefringent plate 7 for separating the light that has been converted into the circularly polarized light by the second quarter-wave plate 6 into linearly polarized light beams that are orthogonal to the directions of separation by the first birefringent plate 5; and a projection lens 8 for projecting the light beams that have been separated by the second birefringent plate 7 on a screen in a magnified state. According to this configuration, each of the pixels is projected as four spatially separated pixels on a projected image, thereby allowing a pixel grid as ineffective portions of the pixels to be made inconspicuous.
As examples of a pattern in which the respective pixels are separated spatially on a projected image to allow the pixel grid to be made inconspicuous, the following patterns are conceivable: a parallel two-point separation pattern in which light is separated into two light beams that are spatially separated in the horizontal direction or the vertical direction; an oblique two-point separation pattern in which light is separated into two light beams that are spatially separated in an oblique direction; an oblique four-point separation pattern in which light is separated into four light beams that are spatially separated in oblique directions; a square four-point separation pattern in which light is separated into four light beams that are spatially separated in the horizontal direction and the vertical direction; and the like. When the numerical aperture is small and the ineffective portions of respective pixels have a large area as in the case of a transmission liquid crystal light valve, the ineffective portions cannot be covered sufficiently by employing either of the above-described two-point separation patterns, so that an effect of making the pixel grid inconspicuous cannot be exhibited sufficiently. On the other hand, when the oblique four-point separation pattern is employed, vertical lines or horizontal lines on a projected image are jagged, resulting in deteriorated image quality. On this account, the square four-point separation pattern is most suitable as a pattern in which the respective pixels are separated spatially on a projected image to allow the pixel grid to be made inconspicuous. Since light emitted from a liquid crystal light valve is polarized linearly, the square four-point separation pattern conventionally has been achieved by using the first quarter-wave plate 4, the first birefringent plate 5, the second quarter-wave plate 6, and the second birefringent plate 7, as shown in FIG. 9. In this case, as a material of the birefringent plates, quartz is used, which is a uniaxial optical crystal that absorbs a small amount of light and is excellent in uniformity. As a material of the quarter-wave plates, quartz or an oriented film is used.
However, this configuration requires the use of either four pieces of quartz or two pieces of quartz and two oriented films, resulting in an increase in cost.